By reason of great altitude and extreme aridity the rocks are rarely masked by subaerial gravels, soil, or vegetation, and the book of geology lies open. John Wesley Powell on the Uinta Mountains, 1876

It’s a common analogy – rocks as book. If we can read the rocks, we can learn their age, where and how they came to be, what was happening then, and maybe something of what’s happened since. In theory, ancient rocks tell ancient stories – many hundreds of millions or even billions of years old. But those chapters are often hard to read, confusing, incomplete, or largely unintelligible.

John Wesley Powell called the Uinta Mountains an open geological book, but actually he and the other pioneering geologists of the Great Surveys found some chapters to be tough reads – especially the purplish-red sandstones that dominate the range, from the high country down to the canyons cut by the Green River. The three survey geologists that worked in the Uintas all misinterpreted them as being mid-Paleozoic in age. They’re actually much older – Precambrian. The error is understandable. In the 1870s, geology was still a young science with limited tools and resources. Little was known about the geology of the region and nothing specific to the Uinta Mountains.

[For more about the great geologists’ struggles with the enigmatic sandstone (and each other) see my previous post.]

Uinta Mountains (southwest Wyoming and northeast Utah) on Ferdinand Vandeveer Hayden's General Geologic Map of the Area Explored and Mapped ...1869-1880; from Special Collections, University of Wyoming Libraries.

I made the same error: “Precambrian? ... nah, looks like sandstone ... maybe the guidebook mileage is wrong.”

These sandstones are now included in the Uinta Mountain Group (UMG), along with occasional layers of quartzite, shale and conglomerate. It occurs only in the Uinta Mountains, where it’s exposed in an east-west zone more than 100 miles long. Its great thickness can be measured in kilometers – as many as seven! On the broad crest of the range, the strata lie nearly horizontal (Dehler and Sprinkel 2005).

Their thickness, as observed in different parts of the range, is from 10,000 to 12,000 feet, while, as their base is never reached, the actual thickness of the formation may be indeﬁnitely greater. Samuel Franklin Emmons (King Survey), 1877

"Glacial lake in the summit region of the Uinta Mountains." Photo by TH O'Sullivan, US Geological Exploration of the 40th Parallel (King Survey); USGS.

The geologists of the Great Surveys were very much disappointed to find that the Uinta sandstones contained no fossils. They were compiling geologic maps, the first for the region, and needed to assign ages to the various rock units. In the absence of fossils, they had to infer an age for the Uinta sandstones based on position relative to other strata, and did so reluctantly.

... the Uinta Sandstone may be considered Devonian—an opinion which I would yield upon the slightest paleontologic evidence to the contrary. John Wesley Powell, 1876

In fact there is paleontologic evidence to the contrary – the UMG contains fossils, but in the 1870s they couldn’t be seen. Cyanobacteria and other microfossils in shale layers, combined with zircon dating, have put the age of the UMG at 850-720 million years – middle Neoproterozoic (Dehler and Sprinkel 2005). Thus modern geologists have come up with a fairly precise age for the UMG ... but so what? If all we learned from rocks were their ages, the book of geology would be really boring. Fortunately they’ve figured out enough to know that the UMG is quite an exciting chapter.

Precambrian Uinta Mountain Group in fore- and mid-ground. A Laramide-age fault brought it into contact with the Jurassic Nugget sandstone (ridge on skyline).

First there are the sandstones themselves. Their structure and texture hint at the environments where the sediments were deposited. In the high country of the western Uintas, outcrops include repetitive sequences of progressively finer deposits, suggesting repeated marine transgressions (rising seas) (Kingsbury-Stewart et al. 2013).

One of these remarkable structures stands out isolated from the rest, in the middle of the valley of Smiths Fork, which was estimated to rise 1,500 feet above its base, and so much like a Gothic church did it appear that members of the Survey gave it the name of Haydens Cathedral. View shows with remarkable clearness their horizontal stratification. They are as regular as the steps of a pyramid, and as the snow rests upon each step, it relieves them in the strongest manner. William Henry Jackson, 1870; USGS.

Fine-scale cross-bedding and other evidence of streams, deltas and shorelines are common too, more so in the eastern part of the range. The UMG has been described as a mix of stream, coastal and marine deposition. It appears the western part of the Uintas was once a stream-fed marine basin, with braided river systems, deltas and tidal flats to the east (Dehler and Sprinkle 2005).

UMG memento with fine-scale cross-bedding.

Another clue lies on the north side of the Uinta Mountains – a possible east-west normal fault contemporary with the UMG. Maybe the south block dropped down to form a basin where sediments of the UMG accumulated. Indeed, the UMG thickens northward suggesting a half-graben (basin) dipping to the north, toward the old fault.

And then there’s the timing. UMG rocks are approximately 850-720 million years old ... that’s when the great supercontinent Rodinia was coming apart, including in the vicinity of today’s Uinta Mountains.

Breakup and dispersal of the Rodinia supercontinent in a 750 Ma reconstruction. My purple arrow indicates rough location of today’s Uinta Mountain Group. From The Encyclopedia of Earth.

Put the clues together – marine and fluvial (stream) deposition, a basin formed by a normal (extensional) fault, the supercontinent unraveling nearby – and you can see why geologists think UMG sediments were deposited in a rift valley, a growing tear in Rodinia. For more than a hundred million years, seas invaded from the west and rivers flowed in from the east and north, depositing silt, gravel and massive amounts of sand in the growing basin.

A possible reconstruction of Rodinia during rifting 770 - 740 million years ago. The Uinta rift valley was invaded from the west by a shallow sea. Rivers entered from the north and east. From Kingsbury-Stewart et al. 2013, greatly simplified.

The Uinta rift didn’t tear all the way. It may have been the failed arm of a triple junction. It’s thought that triple junctions – three radiating rifts – form when a continent is breaking up. Two of the arms split completely, but the third fails. In this case, the continent broke up along the north and south arms, contributing to the birth of the Pacific Ocean. But the Uinta arm failed, and Wyoming and Utah stayed together.

Roughly 1.1 billion years ago, a large continental plate began breaking up. Geological forces pulled the plate apart in three directions forming a series of 3-pointed stars. The center of one of these stars was close to where Salt Lake City is today and the eastern arm of the this star is in this general area [of the sign].

Over time this east-west basin filled with sediments to 23,000 feet deep. The sediments, brought down by rivers and streams and an occasional flooding from the ocean, hardened into the reddish Precambrian rock ... the Uinta Mountain Group State of Utah, interpretive sign at turnoff to Sheep Creek Canyon Geological Area.

Hayden, Powell, Emmons and King ponder today's interpretation of their old nemesis, the Uinta sandstone.

Postscript

It was just recently (only 70 to 40 million years ago) that the Uinta Mountains formed. They were uplifted during the Laramide orogeny, the mountain-building event that produced the Rocky Mountains. Western North America was being compressed, probably from the west, and most Laramide mountain ranges trend roughly north to south. But the Uintas are different.

The Uinta Range ... is the only considerable mountain uplift within the limits of our exploration and indeed with few exceptions in the whole Cordilleran system of the United States which has an east and west trend.Samuel Franklin Emmons, 1877

Hmmm ... the failed Precambrian rift had an “east and west trend” too ... is this more than a coincidence? Some geologists think the old failed rift provided a zone of weakness along which the Uinta Mountains rose when the continent was squeezed 700 million years later (e.g. Hurst 2010). Others look even further back, suggesting that a continental suture dating from 1.7 billion years ago – the Cheyenne Belt – provided the zone of weakness, facilitating the Uinta rift late in the Precambrian, and then uplift of the Uintas in the late Cretaceous (e.g. Sprinkel 2014). But as with many things Precambrian, the evidence is confusing and incomplete. Geologists are still struggling to read the ancient texts.

"Gilberts Peak, Uinta Mountains. A beautiful and instructive view of one of the highest peaks in the Uinta Range ... The very plainly marked strata of red sandstones and quartzites incline very slightly to the southeast." Photo by WH Jackson, 1870; USGS.

the view of one of these mountain-lakes, with its deep-green water and fringe of meadow-land, set in the sombre frame of pine forests, the whole enclosed by high walls of reddish-purple rock, whose horizontal bedding gives almost the appearance of a pile of Cyclopean masonry, forms a picture of rare beauty. Samuel Franklin Emmons, 1877